Filter medium and a use thereof

ABSTRACT

The present invention relates to a filter medium being at least formed of a pre-filter substrate laminated with a fine-filter substrate by means of a third binder, wherein the pre-filter substrate comprises synthetic fibers and a first binder, the pre-filter substrate working as a combined surface and depth filter, and the fine-filter substrate comprises at least a second binder and one of synthetic fibers and inorganic fibers.

The present invention relates to a filter medium for use in bothtransportation and industrial filtration. Specifically, the inventionrelates to a filter medium that is efficient in the removal of solidparticles from gases utilizing both surface and depth filtration andcapable of being pulse-cleaned.

FIELD OF THE INVENTION

There is an increasing need for filtering impurities from theatmosphere, vapors and liquids, i.e. from all kinds of fluids.Especially, both transportation filtration and industrial airfiltration, and specifically the fields of gas turbines, internalcombustion en gines and Air Pollution Control (APC) require filter mediathat are efficient in terms of particle removal. Moreover, indoor airquality is becoming important as respiratory illnesses, allergy andasthma symptoms occur with increasing frequency in industrializedcivilization. While heating, ventilation and air conditioning (HVAC)filters can provide high particle removal capability, these filters alsocreate significant air flow resistance. As a result, high efficiencyHVAC filter systems require powerful fans to move air. Likewise, priorart filter media for air intake applications in gas turbines andtransportation lead to an increased energy consumption. Furthermore, inaddition to air filtration, there is a need to filter impurities fromliquids, like for instance, engine oils, hydraulic oils etc.

BACKGROUND OF THE INVENTION

Traditionally, in industrial filtration, a so-called flow-through filteris used, the operation of which is based on leading the fluid to befiltered more or less at right angles through the filter medium till themedium gets clogged, or rather, the pressure loss across the filtermedium grows to such an extent that the filter medium has to bereplaced. Such prior art filter media use inorganic and synthetic fibersfor the filtration of, e.g. air. A basic drawback of such filters is thefact that the filters' energy consumption increases over time becausetheir resistance to air flow increases with the amount of particleswhich are removed from the air.

One way to control the energy consumption is to reduce the changeinterval of the filters so that the resistance to air flow does not growtoo high. However, this kind of a practice increases the expensesrelated to both service and replacement of the filter. A way to reducethe replacement interval is to use, in place of planar filter elements,pleated filter elements by means of which the effective filter area isincreased.

There are two basic types of flow-through filters, i.e. one applyingsurface filtration, and another applying depth filtration. Surfacefiltration refers to a process where the surface of the filter medium isso dense that the solids separated from the fluid remain on the surfaceof the filter. In depth filtration the filter medium has pores intowhich solid particles get trapped. Often the filter has pores ofvariable sizes such that the pore size gets the smaller the deeper inthe filter the pore is located. Such a filter is oftentimes referred toas a gradient structure filter or a gradient pore structure filter. Inpractice, it means that solids of larger size are trapped in the porescloser to the entrance face of the filter and smaller solids travel alonger way deeper inside the filter medium until they are trapped.Micro-glass fibers are most often considered as the optimal material forthe depth filters, especially in HVAC- types of filters. However,filters or filter elements made of micro-glass are, by nature, fragile,i.e. they need to be handled gently and they cannot be cleaned, wherebythey have to be replaced periodically, and the replacement interval isrelatively short. Naturally, there are also products applying bothsurface and depth filtration.

A way to increase the replacement interval, or lifetime, of filters invarious industrial applications is the use of pulse-cleaning. In suchapplications the filter device is provided with means for periodicallyblowing compressed fluid through the filter in the direction oppositethe normal filtering direction, whereby the fluid pulses flush most ofthe solids trapped in the pores of the filter away from the filter. Thepulse-cleaning may be easily automated, whereby it saves manpower aswell as other service and maintenance expenses. However, sincepulse-cleaning uses compressed fluid it means considerable mechanicalstress on the filter, whereby only a few types of pulse-cleanable filterelements may be found on the market. One of such types is a membranefilter discussed, for instance in WO-A1-2009152439, which has a membranelayer on a support surface, the membrane layer facing the incomingfluid. Another type is a filter having a nanofiber surface facing theincoming fluid. Both filters are pulse-cleanable but they function onlyas a surface filter, i.e. they do not have any pores to collect thesolids but the solids remain on the surface of the filter. Therefore thepressure difference over the filter grows quickly and the need forpulse-cleaning is almost instantaneous. It is a feature characteristicto both the membrane and nanofiber filters that they lose theirfiltering capability very suddenly, in an unexpected manner, even whenpulse cleaned, whereby the replace interval thereof is unpredictable.Furthermore, both nanofibers and membranes are expensive materials,whereby already the market price of such a filter is high, not tomention the various costs, including continuous condition monitoring,related to the needs of sudden replacement thereof.

Industrial filtration, like for instance gas turbine applications,require the use of HEPA-filters (HEPA=High Efficiency Particulate Airfilter). Prior art knows two types of HEPA-filters. The first type is aflow-through filter based on the use of a high amount of micro-glass.The filter acts as a depth filter but is not a pulse-cleanable one dueto its simple structure and high micro-glass content whereby itsreplacement interval is relatively short. The second HEPA- filter typeis a membrane filter (WO-A1-2009152439) where the filtration takes placethrough a PTFE (polytetrafluoroethylene)-film as surface filtration. Themembrane filter is, as already mentioned, a pulse-cleanable one, but asit is a surface filter, the openings in the surface of the filter getclogged suddenly, irrespective of the pulse-cleaning, whereby thereplacement interval is hard to predict.

BRIEF DESCRIPTION OF THE INVENTION

The present invention aims at solving at least one of the abovediscussed problems.

An object of the present invention is to introduce a depth-typepulse-cleanable HEPA-filter, which maintains its filtering capabilityfor a longer time period than HEPA-filters of prior art, i.e. which doesnot clog suddenly, in an unexpected manner.

Another object of the present invention is to introduce apulse-cleanable HEPA-filter, the structure of which allowspulse-cleaning unlike prior art HEPA-filters, i.e. the filter medium isstrong enough to endure cleaning pulses in a direction opposite to itsnormal operating direction.

Yet another object of the present invention is to introduce an improvedpulse-cleanable filter medium that increases the efficient lifetime ofan industrial filter medium.

Still another object of the present invention is to introduce animproved pulse-cleanable HEPA-filter medium, the replacement intervalthereof being predictable.

The present invention therefore relates to a pulse-cleanable filtermedium for filtering a fluid as defined in claim 1.

Other features characteristic to the present invention will becomeapparent from the dependent claims.

The present invention brings about a number of advantages of which atleast the following may be mentioned

-   -   filter medium of the present invention may be used in both        pulse-cleanable and non-pulse-cleanable applications,    -   filter medium of the present invention may be easily adjusted        for a number of different HEPA-grades,    -   highly repellent against humidity,    -   HEPA-filter with optimal depth or surface filtration properties        depending on media combination,    -   pulse-cleanable filter that is made of low-cost materials,    -   long lifetime,    -   low maintenance and replacement costs, and    -   no unexpected replacements, predictable lifetime.

DEFINITIONS

Bicomponent fibers or, more generally, multi-component fibers—the termrefers to fibers formed from at least two polymers forming one fiber. Asa particular example of a multi-component fiber, a “bicomponent fiber”includes two polymers arranged in substantially constantly positioneddistinct zones across the cross-section of the bicomponent fiber andextend continuously along the length of the bicomponent fiber. Theconfiguration of such a bicomponent fiber may be, for example, asheath/core configuration wherein one polymer is surrounded by anotheror may be a side-by-side configuration or an “islands-in-the-sea”configuration. For two-component fibers, the polymers may be present inratios of 75/25, 50/50, 25/75 or in any other desired ratio. Bicomponentfibers have usually a low melting point sheath and a higher meltingpoint core, whereby their main function is bonding other fibers to forma mechanically strong substrate. Bicomponent fibers are normallynominated in the following manner PET/co-PET, PET/co-PE (PE=polyethylene), PET/co-PP (PP =polypropylene) or PET/co-PA, where theformer mentioned polymer forms the sheath and the latter one the core.More generally, the bicomponent fibers may be made from a variety ofthermoplastic materials including polyolefins (such as polyethylenes,polypropylenes), polyesters (such as polyethylene terephthalate,polybutylene terephthalate, PCT), nylons including nylon 6, nylon 6,6,nylon 6,12, etc. Any thermoplastic that can have an appropriate meltingpoint can be used in the low melting component of the bicomponent fiber.The polymers of the bicomponent (core/shell or sheath and side-by-side)fibers are made up of different thermoplastic materials, such as forexample, polyolefin/polyester (sheath/core) bicomponent fibers wherebythe polyolefin, e.g. polyethylene sheath, melts at a temperature lowerthan the core, e.g. polyester. Typical thermoplastic polymers includepolyolefins, e.g. polyethylene, polypropylene, polybutylene, andcopolymers thereof, polytetrafluoroethylene, polyesters, e.g.polyethylene terephthalate, polyvinyl acetate, polyvinyl chlorideacetate, polyvinyl butyral, acrylic resins, e.g. polyacrylate, andpolymethylacrylate, polymethyl-methacrylate, polyamides, namely nylon,polyvinyl chloride, polyvinylidene chloride, polystyrene, polyvinylalcohol, polyurethanes, cellulosic resins, namely cellulosic nitrate,cellulosic acetate, cellulosic acetate butyrate, ethyl cellulose, etc.,copolymers of any of the above materials, e.g. ethylene-vinyl acetatecopolymers, ethylene-acrylic acid copolymers, styrene-butadiene blockcopolymers, Kraton rubbers and the like.

Binder—general term for means used for bonding various fibers to oneanother. Includes liquid and solid binder resins. The solids comprisingpowders, granules, fully meltable fibers, and partially meltable fibers(bicomponent and multicomponent fibers).

Binder fiber—general term for fibers comprising fully meltable fibersand bi- and multicomponent fibers.

Binder resins—general term for substances bonding various fibers into amechanically stable substrate. May be divided into thermoplastic andthermosetting resins. The thermoplastic binder resin materials may beused in the form of a dry powder, granule or liquid. Typically they areaqueous dispersions of vinyl thermoplastic resins. Resin used as bindermay be in the form of water soluble or dispersible polymer addeddirectly to the fiber suspension or in the form of thermoplastic binderfibers of the resin material intermingled with the fiber suspension tobe activated as a binder by heat applied after the substrate is formed.Resins include vinyl acetate materials, vinyl chloride resins, polyvinylalcohol resins, polyvinyl acetate resins, polyvinyl acetyl resins,acrylic resins, methacrylic resins, polyamide resins, polyethylene vinylacetate copolymer resins, thermosetting such as urea phenol, ureaformaldehyde, melamine, epoxy, polyurethane, curable unsaturatedpolyester resins, polyaromatic resins, resorcinol resins and similarelastomer resins. The preferred materials for the water soluble ordispersible binder polymer are water soluble or water dispersiblethermosetting resins such as acrylic resins, methacrylic resins,polyamide resins, epoxy resins, phenolic resins, polyureas,polyurethanes, melamine formaldehyde resins, polyesters and alkydresins, generally, and specifically, water soluble acrylic resins,methacrylic resins, polyamide resins, that are in common use in theindustry. Such binder resins typically coat the fiber and adhere fiberto fiber in the final non-woven matrix. Sufficient resin is added to thefurnish to fully coat the fiber without causing any film to cover thepores formed in the sheet, media, or filter material. The resin may beadded to the furnish during the non-woven web formation or may beapplied to the web after the formation thereof.

Cross Machine Direction (CD or XMD)—the direction perpendicular to thedirection in which the fibrous web travels as it is forming.

DENS—Filtering Efficiency—Standard BS EN 1822 describes the factorytesting of the filtration properties of absolute filters i.e. EfficientParticulate Air (EPA)-, High Efficiency Particulate Air (HEPA)- andUltra Low Penetration Air (ULPA)- filters. The filters are divided in anumber of groups based on the filtration efficiency thereof. Thefiltration efficiency is determined by using Di-Ethyl-Hexyl-Sebacat(DENS) droplets as the particles to be filtered. The filtrationefficiency indicates the percentage of particles trapped by the filtercompared to the full amount of particles in the flow to be filtered. Thefilter groups and their required minimum filtration efficiencies are asfollows:

-   -   E10-≥85%,    -   E11-≥95%,    -   E12-≥99.6%,    -   H13-≥99.95%,    -   H14-≥99.995%,    -   U15-≥99.9995%,    -   U16-≥99.99995%, and    -   U17-≥99.999995%.

Depth filtration—refers to a process where the filter medium has poresof variable sizes such that the pore size gets the smaller the deeper inthe filter the pore is located, oftentimes such a filter structure isreferred to as gradient structure filter or gradient pore structurefilter.

Finess—fiber or tow thickness, normally given in deniers.

Inorganic fibers—fibers made of inorganic materials, like for instanceglass, carbon, silica.

Machine Direction (CD or XMD)—the direction in which the fibrous webtravels as it is forming.

Meltable fibers—fibers melting fully when heated, used for bondingfibers to one another and made of, for instance, PE (polyethylene), PP,polyvinyl chloride (PVC) and polyvinyl alcohol (PVA).

Pulse-cleanable—the term refers to cleanability of a filter medium bymeans of a fluid jet in a direction opposite the flow direction of thefluid to be cleaned. A test procedure used for determining thecleanability of the filter medium is discussed in VDI 3926-2004, Testingof cleanable filter media, Standard test for the evaluation of cleanablefilter media. ISO 11057:2011 also discusses a standard reference testmethod for the comparative characterization and evaluation of pulse-jetcleanable filter media.

Surface filtration—refers to a process where the surface of the filtermedium is so dense that the solids separated from the fluid remain onthe surface of the filter.

Synthetic fibers—fibers made, for instance, of polyethyleneterephthalate (PET), polybutylene terephthalate (PBT), polypropylene(PP), polyamide (PA), acrylic, lyocell, rayon, aramid, nylon,polyolefin, polyester, or viscose.

DETAILED DESCRIPTION OF THE INVENTION

Generally speaking the filter medium of the present invention is made ofat least two separately manufactured filter substrates, i.e. apre-filter substrate and a fine-filter substrate, laminated together toform a pulse-cleanable filter medium, or a fine-filter substrate laid ona separately manufactured pre-filter substrate to form a pulse-cleanablefilter medium.

Pre-filter Substrate

The pre-filter substrate comprises at least synthetic fibers likepolyethylene terephthalate (PET), polybutylene terephthalate (PBT),polypropylene (PP), polyamide (PA), acrylic, lyocell, rayon, aramid,nylon, polyolefin, polyester, or viscose fibers having an averagediameter ranging from 0.1 to 100 micrometers, preferably ranging from0.1 to 50 micrometers, more preferably ranging from 0.1 to 30micrometers and an appropriate first binder. A part of the syntheticfibers may be replaced with inorganic fibers, like micro-glass, carbonor silica fibers having an average diameter ranging from 0.01 to 5micrometer. A preferred choice for the first binder is multicomponentfibers, preferably, but not necessarily, bicomponent fibers like forinstance PET/co-PET, PET/co-PE, PET/co-PP, or PET/co-PA fibers (see moredetailed information in ‘Definitions’ above), having a finess rangingfrom 1 to 15 den, preferably about 2.2 den, 4 den or 6 den, i.e. thediameter being of the order of 8-25 μm, and after curing, resulting fromthe melting of the sheath, naturally less. Another option, in additionto or in place of the bi- or multi-component fibers, for the firstbinder is binder resin, such as at least one of acrylic resins, alkydresins, butadiene resins, epoxy resins, melamine formaldehyde resins,melamine resins, methacrylic resins, phenolic resins, polyamide resins,polyaromatic resins, polyester resins, polyethylene vinyl acetatecopolymer resins, polyurea resins, polyurethane resins, polyvinylacetate resins, polyvinyl acetyl resins, polyvinyl alcohol resins,polyvinylchloride (PVC) resins, resorcinol resins, styrene resins, ureaphenol resins, urea formaldehyde resins, vinyl acetate resins and vinylchloride resins or any combination thereof that may be used in bondingthe fibers to one another. In place of or in addition to the bi- ormulticomponent fibers and/or the above listed binder resins it is alsopossible to use, as at least a part of the first binder, fully meltablefibers like for instance at least one of PE (polyethylene), PP,polyvinyl chloride (PVC) and polyvinyl alcohol (PVA) fibers.

The first binder, preferably, but not necessarily, the bicomponentfibers provided in the pre-filter substrate, ensures most of themechanical strength and integrity of the filter, and makes it possiblethat pulse-cleaning, i.e. a powerful jet of compressed fluid blown in adirection against the ordinary working direction of the filter, may beapplied for cleaning the filter. Additionally, the pre-filter substratecontains, in at least a majority of the embodiments of the presentinvention, micro-glass fibers, which give both fine-filter capabilityand mechanical strength to the substrate.

It is a feature common to all embodiments of the present invention thatdepending on the type of the pre-filter, its contribution to the finalDEHS-efficiency may vary. In other words, the pre-filter substrateprovides the HEPA-filter with most of its mechanical strength, i.e.ensures the pulse-cleanable structure of the filter, and with a part ofthe DEHS-efficiency of the entire filter, whereas the differences in theDEHS-efficiency of the filter are mostly provided by the fine-filtersubstrate.

Thus, the pre-filter substrate of the present invention has a basisweight of 20 g/m²-80 g/m², preferably about 30 g/m²-70 g/m², morepreferably about 35-50 g/m². The basis weight is measured in accordancewith ISO 536:2012, Paper and board,—Determination of grammage. Thepre-filter substrate, generally, in accordance with a first preferredembodiment of the present invention, comprises 20-95 wt-%, preferably25-80 wt-%, more preferably about 25-50 wt-% synthetic fibers; 0-50wt-%, preferably 20-40 wt-%, more preferably about 30-35 wt-%bicomponent fibers; 0-30 wt-%, preferably 10-25 wt-%, more preferablyabout 15-20 wt-% micro-glass fibers; and, optionally, 0-20 wt-%,preferably about 0-15 wt- %, more preferably about 2-10 wt-% binderresin or fully meltable fibers. The wt-% throughout the specificationrefers to the dry amount of the components forming the intermediatesubstrate or the end product. i.e. the pulse-cleanable filter medium.

In accordance with a second preferred embodiment of the presentinvention the pre-filter substrate comprises 20-95 wt-% synthetic fibersand 5-80 wt-% first binder.

In accordance with a third preferred embodiment of the present inventionthe pre-filter substrate comprises 25-80 wt-%, preferably about 35-50wt-% synthetic fibers; 20-75 wt-%, preferably about 40-55 wt-% bi- ormulticomponent fibers; and 0-15 wt-%, preferably 2-10 wt-% binder resinor fully meltable fibers.

In accordance with a fourth preferred embodiment of the presentinvention the pre-filter substrate comprises 55-85 wt-%, preferably60-80 wt-% synthetic fibers; 10-30 wt-%, preferably about 15-25 wt-%micro-glass fibers; and 2-15 wt-%, preferably about 2-10 wt-% binderresin or fully meltable fibers.

In accordance with a fifth preferred embodiment of the present inventionthe pre-filter substrate comprises 25-60 wt-%, preferably 30-60 wt-%,more preferably about 40-50 wt-% synthetic fibers; 20-45 wt-%,preferably 30-40 wt-% bicomponent fibers; 10-30 wt-%, preferably about15-20 wt-% micro-glass fibers; and 0-20 wt-%, preferably about 0-15wt-%, more preferably about 2-10 wt-% binder resin or fully meltablefibers.

In accordance with a sixth preferred embodiment of the present inventionthe pre-filter substrate contains two types of synthetic fibers,preferably PET-fibers, i.e. 3-35 weight-%, preferably 33 weight-%PET-fibers with a diameter of 6 pm and 5-15 wt-%, preferably 10 wt-%PET-fibers with a diameter of 10 pm; 30-40 wt-% bicomponent fibers; twotypes of micro-glass fibers, i.e. 10-15 wt-%, preferably 13 wt-%micro-glass fibers with a diameter of 0.1-1 μm and 1-9 wt-%, preferably5 wt-% micro-glass fibers with a diameter of 0.1-0.6 μm diameter 5%, and5 wt-% acrylic binder.

In accordance with a seventh preferred embodiment of the presentinvention any one of the pre-filter substrates discussed above may beformed by using a wet-laid technology, i.e. the various differentcomponents forming the pre-filter substrate are mixed into a liquid,preferably water, to form a fiber suspension or furnish. The firstbinder, if in appropriate form, i.e. liquid or solid, may be added tothe furnish or applied through the production process with anapplication of low impact to the fiber structure. The furnish is, afterbeing mixed to a homogenous mixture, laid from a headbox on a liquidpervious wire for forming a fibrous nonwoven web. The wet-laying of thefilter substrate has the advantage that, as a result of different speedto settle to form a web, the concentration of the finer and heaviermicro-glass fibers, if present in the furnish, is at its highest closeto the wire whereas the other, lighter fibers have their higherconcentrations farther away from the wire. In practice, this means thatthere is a porosity gradient in the filter substrate, i.e. the pores inthe filter substrate reduce in size while approaching the wire-side ofthe substrate. Therefore, the filter substrate is capable of performingso called depth filtration. The same phenomenon may still take placeeven if there were no micro-glass fibers as, in the furnish, there maybe fibers having different properties, such as specific gravity, fiberlength, etc. in view of settling in the web.

In accordance with a further variation of the seventh preferredembodiment of the present invention the headbox used in the productionof the pre-filter substrate is divided in vertical direction into twoseparate chambers each having a slice opening of its own. Both chambersof the headbox are provided with a fiber suspension or a furnish of itsown, whereby the actual pre-filter substrate is formed into a fibrousnonwoven web having two different layers entangled to one another. Thebasic principles of laying more than one layer of furnish in making aweb on a wire is discussed in, for instance, U.S. Pat. No. 3,598,696.When producing a two-layer substrate by means of a wet-laid method, thefurnish for the first layer formed on the wire, i.e. the lower layer,may, for instance, comprise 10-45 wt-%, preferably 10-20 wt-% syntheticfibers; 0-30 wt-%, preferably 10-20 wt-% bi- or multicomponent fibers;and 0-10 wt-%, preferably 0-5 wt-% first binder resin and/or fullymeltable fibers. The second furnish for the second layer wet-laid on thefirst layer may, for instance, comprise 15-30 wt-%, preferably 15-25wt-% synthetic fibers; 0-20 wt-%, preferably 10-20 wt-% bi- ormulticomponent fibers; 10-30 wt-%, preferably 15-20 wt-% micro-glass;and 0-10 wt-%, preferably 0-5 wt-% first binder resin and/or fullymeltable fibers. The above percentage values refer to the dry weight ofthe entire substrate. The synthetic fibers may be of the same quality,for instance 6 or 10 micrometer PET- fibers, or a mixture of fibershaving different diameters. The micro-glass fibers have an averagediameter of 0.01-5 micrometer, a majority, i.e. more than 60% of thefibers having an average diameter between 0.1 and 2 micrometer. However,as the micro-glass fibers are commercially available in various grades(0.4, 0.6, 0.8 etc. or 0.1 to 0.6 micrometers, 0.1 to 0.8 micrometers,0.1 to 1 micrometers, 0.5 to 5 micrometers, etc., referring to theaverage diameter of the fibers in the grade) the micro-glass used in thefurnish may be of only one grade or of a mixture of at least twodifferent grades.

When producing the substrate of the above discussed furnishes in theabove discussed manner the two-layer production ensures that arelatively dense layer of synthetic fibers, and possibly binder fibers,is first arranged on the wire whereafter the micro-glass fibers, ifused, as clearly the heaviest fibers of the various fibers of thefurnish for the upper layer settle on the synthetic fiber orsynthetic-binder fiber layer already present on the wire, and thesynthetic and possibly binder fibers of the second furnish settle on themicro-glass fibers. Thus, when the web, for forming the substrate, isheated and combined to a filter both surfaces of the substrate have astrong synthetic-bicomponent fiber matting, which gives excellentinternal strength for the substrate.

In accordance with a still further variation of the seventh preferredembodiment of the present invention the headbox used in the productionof the pre-filter substrate is divided in vertical direction into threeseparate chambers each having a slice opening of its own, like forinstance discussed in U.S. Pat. No. 3,598,696. Each chamber of theheadbox is provided with a fiber suspension or a furnish of its own,whereby the actual pre-filter substrate is formed into a fibrousnonwoven web having three layers entangled to one another. All threefurnishes may be different, or two furnishes may be the same and onedifferent, whereby, in the latter case, advantageously, the outer layersare formed of the same furnish and the center layer of a differentfurnish. Naturally, also all three furnishes may be the same. Inaccordance with a specific example the two outermost layers of thepre-filter substrate are formed of a furnish comprising at least of afirst binder (preferably bicomponent fibers), and synthetic fibers, andthe center layer at least of a first binder (preferably bicomponentfibers), micro-glass fibers and synthetic fibers.

In accordance with an even further variation of the seventh preferredembodiment of the present invention, when producing a three-layersubstrate by means of a preferred version of the wet-laid methoddiscussed above, the first and third furnishes for the first layerformed on the wire, i.e. the lowest layer, and the third, i.e. theuppermost layer, each comprise 5-30 wt-%, preferably 5-20 wt-% syntheticfibers; 0-20 wt-%, preferably 5-15 wt-% bi- or multicomponent fibers;and 0-10 wt-%, preferably 0-5 wt-% first binder resin and/or fullymeltable fibers. The second furnish for the second layer wetlaid on thefirst layer comprises 15-25 wt-%, preferably 15-20 wt-% syntheticfibers; 0-20 wt-%, preferably 10-20 wt-% bi- or multicomponent fibers;10-30 wt-%, preferably 15-20 wt-% micro-glass fibers; and 0-10 wt-%,preferably 0-5 wt-% first binder resin and/or fully meltable fibers. Theabove percentage values refer to the dry weight of the entire substrate.The synthetic fibers may be of the same quality, for instance 6 or 10micrometer fibers, or a mixture of fibers having different diameters.The micro-glass fibers have an average diameter of 0.01 to 5 micrometer,a majority, i.e. more than 60% of the fibers having an average diameterbetween 0.1 and 2 micrometer. However, as the micro-glass fibers arecommercially available in various grades (0.4, 0.6, 0.8 etc. or 0.1-0.6micrometers, 0.1-0.8 micrometers, 0.1-1 micrometers, 0.5-5 micrometers,etc., referring to the average diameter of the fibers in the grade) themicro-glass used in the furnish may be of only one grade or of a mixtureof at least two different grades.

When producing the substrate of the above discussed furnishes in theabove discussed manner the three-layer production ensures that arelatively dense layer of synthetic and binder fibers is first arrangedon the wire whereafter the micro-glass, as clearly the heaviest fibersof the various fibers of the second furnish for the second layer settleon the synthetic-binder fiber layer already present on the wire, and thesynthetic and binder fibers of the second and third furnishes settle onthe micro-glass fibers. Thus, when the substrate is heated andcompressed both surfaces of the substrate have strong synthetic-binderfiber matting, which gives excellent internal strength for thesubstrate.

In each variation, after being deposited on the wire, water is drainedor drawn out of the fibrous nonwoven web to dry the web to a desireddryness. A clear advantage in using the wet-laying is that in each layera gradient structure is formed, i.e. a structure that allows depthfiltration, as there are always fibers with varying properties in viewof settling in the substrate, whereby certain fibers sink more quickly,and certain fibers more slowly remaining, thus, on top of the substrate.

In accordance with an eighth preferred embodiment of the presentinvention the fibrous nonwoven web may also be air-laid or dry-laid on asurface by using the well-known dry laying methods, like for instance bycarding, airlaying, spunlaid, meltblown or by a combination thereof.

In case the first binder was not added to the mixture of fibers beforethe web-forming, and the use thereof is considered necessary, the firstbinder may be applied on the fibrous nonwoven web by spraying, gravurecoating, impregnation, scattering etc. whereafter the web is heated tomelt the binder fibers and/or the binder resin to bond the variousfibers to one another.

Finally, the pre-filter substrate may be provided, if desired, withmeans for making it repellent against water, humidity, hydro carbonsand/or alcohols. Such chemical means may be formed of or comprisesilicones or fluorochemicals. The chemicals may be arranged on one ormore of the fibers forming the substrate before the forming of thefurnish, or the furnish may be provided with respective liquidchemicals, or the web may be provided with the chemicals by means of,for instance, impregnation or coating.

In view of the above discussed various embodiments of the pre-filtersubstrate it should be understood that any embodiment of the pre-filtersubstrates discussed above may be used with any embodiment of thevarious fine-filter substrates discussed both in the following and inthe later product examples.

Fine-Filter Substrate

The fine-filter substrate comprises a second binder and at least eithersynthetic fibers and/or inorganic fibers. The second binder comprises atleast one of bicomponent fibers, multicomponent fibers, fully meltablefibers and a binder resin. The synthetic fibers are preferably chosenfrom polyethylene terephthalate (PET), polypropylene (PP), PBT,polyamide (PA), acrylic, rayon, aramid, nylon, polyolefin, polyester,lyocell, or viscose fibers having an average diameter ranging from 0.1to 100 micrometers or inorganic fibers. The inorganic fibers are ofmicro-glass, carbon or silica, preferably micro-glass fibers having anaverage diameter ranging from 0.01 to 5 micrometer. The binder resincomprises at least one of acrylic resins, alkyd resins, butadieneresins, epoxy resins, melamine formaldehyde resins, melamine resins,methacrylic resins, phenolic resins, polyamide resins, polyaromaticresins, polyester resins, polyethylene vinyl acetate co-polymer resins,polyurea resins, polyurethane resins, polyvinyl acetate resins,polyvinyl acetyl resins, polyvinyl alcohol resins, polyvinylchloride(PVC) resins, resorcinol resins, styrene resins, urea phenol resins,urea formaldehyde resins, vinyl acetate resins and vinyl chloride resinsor any combination thereof. The fine-filter substrate may, in additionto the above discussed fiber/s, comprise chop-strand glass fibers havingan average diameter ranging from 1 to 30 micrometers.

Generally, in accordance with a ninth preferred embodiment of thepresent invention, the fine-filter substrate of the present inventioncomprises 20-60 wt-%, preferably 20-50 wt-% synthetic fibers; 10-80wt-%, preferably 15-70 wt-% micro-glass fibers and 2-15 wt-%, preferably3-10 wt-% second binder.

In accordance with a tenth preferred embodiment of the present inventionthe fine-filter substrate comprises 35-55 wt-%, preferably 40-50 wt-%synthetic fibers; 10-25 wt-%, preferably 15-20 wt-% micro-glass fibers;20-45 wt-%, preferably 30-40 wt-% bicomponent or multicomponent fibers,and 2-15 wt-%, preferably 3-10 wt-% second binder.

In accordance with an eleventh preferred embodiment of the presentinvention the fine-filter substrate comprises 15-40 wt-%, preferably20-35 wt-% synthetic fibers, 50-80 wt-%, preferably 60-70 wt-%micro-glass fibers and 2-15 wt-%, preferably 3-10 wt-% second binder.

The fine-filter substrate may be produced by any well-known means of webforming. Thus, both air-laid and wet-laid production processes may beused. However, wet laying is the preferred manufacturing choice due toits variability. Thus, all the basic wet-laying methods discussedearlier in connection with the production of the pre-filter substrateare applicable here, too. In other words, the fine-filter substrate maybe manufactured as a single-layer, a two-layer or a multi-layersubstrate resulting in a gradient pore structure filter, i.e. adepth-type filter medium.

EXAMPLE 1—E10 FILTER MEDIUM

The filter medium in accordance with a twelfth preferred embodiment ofthe present invention is designed to fulfil the requirements set by BSEN 1822 for an E10 filter medium, i.e. a filter having the DENSefficiency of at least 85%. The filter medium has a total basis weightof 50 g/m²-150 g/m², preferably about 70-110 g/m². The filter mediumcontains a pre-filter substrate such as discussed already above.

The E10 filter medium contains, as discussed also above, anothersubstrate, i.e. a fine-filter substrate, which is, in this embodiment,as to its manufacture, similar to the above discussed pre-filtersubstrate. The fine-filter substrate may, however, have a somewhatdifferent composition from the above discussed versions of thepre-filter substrate. It is a preferred, but not a necessary, featurefor the desired operation of the fine-filter substrate that the furnishtherefor contains 10-80 wt-%, preferably 10-25 wt-%, more preferably15-20 wt-% micro-glass fibers and at least 5 wt-% second binder in anappropriate form. The second binder may be either the same binderoptionally used in the pre-filter substrate, i.e. at least one ofacrylic resins, alkyd resins, butadiene resins, epoxy resins, melamineformaldehyde resins, melamine resins, methacrylic resins, phenolicresins, polyamide resins, polyaromatic resins, polyester resins,polyethylene vinyl acetate copolymer resins, polyurea resins,polyurethane resins, polyvinyl acetate resins, polyvinyl acetyl resins,polyvinyl alcohol resins, polyvinylchloride (PVC) resins, resorcinolresins, styrene resins, urea phenol resins, urea formaldehyde resins,vinyl acetate resins and vinyl chloride resins or any combinationthereof, or bicomponent fibers, like for instance PET/co-PET, PET/co-PE,PET/co-PP, or PET/co-PA fibers (see more detailed information inDefinitions) or multicomponent fibers or fully meltable fibers, such asat least one of PE (polyethylene), PP, polyvinyl chloride (PVC) andpolyvinyl alcohol (PVA) fibers. The second binder preferably comprises2-15 wt-%, preferably 3-10 wt-% binder or fully meltable fibers, and20-45 wt-%, preferably 30-40 wt-% bicomponent or multicomponent fibers.Furthermore, the furnish comprises 20-60 wt-%, preferably 35-55 wt-%,more preferably 40-50 wt-% synthetic fibers, too. It should be alsounderstood that bi- or multicomponent fibers may be used to replace atleast a part of the binder and the synthetic fibers or vice versa, i.e.by increasing the amount of bi- or multicomponent fibers the amount ofbinder and synthetic fibers may be reduced and by reducing the amount ofbi- or multicomponent fibers the amount of binder and synthetic fibersmay be increased.

As to the fine-filter substrate, it may be formed of a two-layer or athree-layer web as discussed already earlier in connection with thepre-filter substrate.

Finally, the fine-filter substrate may be provided, if desired, withmeans for making it repellent against water, humidity, hydro carbonsand/or alcohols. Such chemical means may be formed of or comprisesilicones or fluorochemicals. The chemicals may be arranged on one ormore of the fibers forming the fine-filter substrate before the formingof the furnish, or the furnish may be provided with respective liquidchemicals, or the web may be provided with the chemicals by means of,for instance, impregnation or coating.

For producing the E10 filter medium the two substrates are laminated toone another by means of, for instance, applying 3-10 g/m², preferablyabout 5 g/m², for instance by gravure coating, curtain coating orspraying, appropriate third binder, i.e. a hot melt binder or adhesive,like for instance ethylene-vinyl acetate (EVA) copolymers, polyolefins(PO), atactic polypropylene (PP or APP), polybutene-1, oxidizedpolyethylene, polyamides and polyesters, styrene block copolymers (SBC),preferably a polyurethane hot melt glue on one of the substrates, andthereafter combining the substrates to form the pulse-cleanable filtermedium. Another way to laminate the two substrates together is to heatat least one of the substrates to make the thermoplastic already thereintacky, and thereafter combine the substrates to form the pulse-cleanablefilter medium. The thermoplastic above comprises both meltable fibersand bi- or multicomponent fibers.

In accordance with a variation of the twelfth preferred embodiment ofthe present invention the wire-sides of the webs, i.e. the sides of thewebs located against the wire during the manufacturing of the webs, arepositioned against one another in the final E10 filter medium. Inaccordance with another variation of the present invention the wire-sideof one web is positioned against the side away from the wire of theother web in the final E10 filter medium. In accordance with a furthervariation of the twelfth preferred embodiment of the present inventionthe wire-sides of the webs are positioned as the outer faces of thefilter medium, i.e. the sides facing away from the wire in theirmanufacturing stage are being positioned against one another in thefinal E10 filter medium. When using the filter medium, it may be used asthe wire side positioned as the dirty side, i.e. against the dirty fluidflow to the filter medium or the wire side as the clean side, i.e.facing away from the dirty fluid flow.

As an exemplary E10 product may be mentioned a filter medium comprisinga pre-filter substrate and a fine-filter substrate. The pre-filtersubstrate is manufactured in accordance with the variation of the abovediscussed seventh preferred embodiment of the present invention by usinga three-layer headbox provided with two different furnishes such thatthe surface layers are of similar furnish and the center layer of thedifferent furnish. The pre-filter substrate comprises 33 wt-% PET-fibershaving a diameter of 6 micrometer, 10 wt-% PET- fibers having a diameterof 10 micrometer, 13 wt-% micro-glass fibers having an average diameterbetween 0.1 and 0.6 micrometer, 5 wt-% micro-glass fibers having anaverage diameter between 0.1 and 1.0 micrometer, 34 wt-% bi-componentfibers having an average diameter between 8 and 25 micrometer, and 5wt-% binder or meltable fibers.

The fine filter substrate is equal with the pre-filter substrate. Thusboth the pre-filter substrate and the fine-filter substrate have atensile strength determined in accordance with standard Scan-P 38:80,i.e. ISO 1924-2: 1994, in Machine Direction (MD) of the order of 600-700N/m, and in Cross Machine Direction (CD) of the order of 400-500 N/m. Ina similar manner both the pre-filter substrate and the fine-filtersubstrate have the same Mullen burst strength determined in accordancewith standard Scan-P 24, Paper and board or ISO 2758. The substrateshave both the dry and the wet Mullen burst strength of the order of100-150 kPa. The mean flow pore size determined by using Porometer 3G(Quantachrome Instruments, Boynton Beach, Fla. U.S.A.) varies between 8and 10 micrometer. The fine-filter substrate has, naturally, the samemean flow pore values.

The tensile strength for the laminated E10 product is in MD-direction ofthe order of 1500-1600 N/m, and in CD-direction of the order of 800-900N/m, the Mullen burst strength (both wet and dry) within 200-400 kPa,preferably within 250-400 kPa, and the mean flow pore size between 5 and6 micrometer.

By means of the above described laminated structure of the filter mediumthe strength properties of the medium are excellent so that the filtermedium tolerates pulse-cleaning, i.e. a powerful fluid jet in thedirection opposite the ordinary working direction of the filter medium.

EXAMPLE 2—E12 FILTER MEDIUM

The filter medium in accordance with a thirteenth preferred embodimentof the present invention is designed to fulfil the requirements set byBS EN 1822 for an E12 filter medium, i.e. the DENS efficiency is atleast 99.5%. The E12 filter medium of the present invention has a totalbasis weight of 50 to 200 g/m², preferably from 80 to 150 g/m², morepreferably 100-130 g/m². The E12 filter medium of the present inventioncontains a pre-filter substrate with a basis weight of 20 g/m²-80 g/m²,preferably 30 g/m²-70 g/m², more preferably 35-50 g/m², corresponding tothe pre-filter substrate discussed already above, and a fine-filtersubstrate having a basis weight of 30 g/m²-180 g/m², preferably from50-120 g/m², more preferably about 60-90 g/m².

The fine-filter substrate is formed of synthetic fibers like forinstance PET, PBT, PP, PA, acrylic, lyocell, rayon, aramid, nylon,polyolefin, polyester, or viscose fibers; micro-glass fibers; and asecond binder. The second binder comprises at least one of bi-componentfibers, multicomponent fibers, fully meltable fibers and binder resins.The binder resins comprise at least one of acrylic resins, alkyd resins,butadiene resins, epoxy resins, melamine formaldehyde resins, melamineresins, methacrylic resins, phenolic resins, polyamide resins,polyaromatic resins, polyester resins, polyethylene vinyl acetatecopolymer resins, polyurea resins, polyurethane resins, polyvinylacetate resins, polyvinyl acetyl resins, polyvinyl alcohol resins,polyvinylchloride (PVC) resins, resorcinol resins, styrene resins, ureaphenol resins, urea formaldehyde resins, vinyl acetate resins and vinylchloride resins or any combination thereof. The fine-filter substratecontains 10-80 wt-%, preferably 50 to 80 wt-%, more preferably 60-70wt-% micro-glass fibers; 20-60 wt-%, preferably 15-40 wt-%, morepreferably 20-35 wt -% synthetic fibers; and 2-15 wt-%, preferably about3-10 wt-% second binder (including powder, granule or liquid binders).As to the second binder it should be understood that also bi-ormulticomponent fibers may be used. However, in such a case the amount ofboth the synthetic fibers and the binder should be reduced. For exampleby replacing one half of the synthetic fibers with bi- or multicomponentfibers also the amount of the binder could be about halved. Themicro-glass fibers have an average diameter of 0.01 to 5 micrometer, amajority, i.e. more than 60% of the fibers having an average diameterbetween 0.1 and 2 micrometer. However, as the micro-glass fibers arecommercially available in various grades (0.4, 0.6, 0.8 etc. or 0.1-0.6micrometers, 0.1-0.8 micrometers, 0.1-1 micrometers, 0.5-5 micrometers,etc., referring to the average diameter of the fibers in the grade) themicro-glass used in the furnish may be of only one grade or of a mixtureof at least two grades.

In accordance with a preferred variation of the thirteenth embodiment ofthe present invention, micro-glass fibers from at least two differentgrades, or diameter ranges, is used, for instance about 10-20 wt-% ofthe micro-glass originating from a grade having an average diameter of 1to 3 micrometers, and the rest (90-80 wt-%) originating from a gradehaving an average diameter of 0.1 to 1.0 micrometer. Preferably, thesynthetic fibers are PET-fibers having an average diameter of 10 to 15micrometers.

The web for the fine-filter substrate may be produced with any knownpapermaking-type machines such as commercially available fourdrinier,wire cylinder, Stevens former, rotoformer, inver former, venti former,and inclined delta former. After the fibrous web for the fine-filtersubstrate is dried to an appropriate dryness second binder, unless addedto the furnish for making the fine-filter substrate, is applied on theweb by spraying, gravure coating, impregnation, scattering etc.whereafter the web is heated to bind the various fibers to one another.

Finally, the fine-filter substrate may be provided, if desired, withmeans for making it repellent against water, humidity, hydro carbonsand/or alcohols. Such chemical means may be formed of or comprisesilicones or fluorochemicals. The chemicals may be arranged on one ormore of the fibers forming the fine-filter substrate before the formingof the furnish, or the furnish may be provided with respective liquidchemicals, or the web may be provided with the chemicals by means of,for instance, impregnation or coating.

For producing the E12 filter medium the two substrates, i.e. thepre-filter substrate and the fine-filter substrate are laminated to oneanother by means of applying, for instance by gravure coating, curtaincoating or spraying 3-10 g/m², preferably about 5 g/m², appropriatethird binder, i.e. hot melt binder or adhesive, like for instanceethylene-vinyl acetate (EVA) copolymers, polyolefins (PO), atacticpolypropylene (PP or APP), polybutene-1, oxidized polyethylene,polyamides and polyesters, styrene block copolymers (SBC), preferably apolyurethane hot melt glue on one of the substrates, and there-aftercombining the substrates to form the pulse-cleanable filter medium.Another way to laminate the two substrates together is to heat at leastone of the substrates to make the thermoplastic already therein tacky,and thereafter to combine the substrates to form the pulse-cleanablefilter medium. The thermoplastic here refers to either meltable or bi-or multicomponent fibers.

In accordance with another variation of the thirteenth preferredembodiment of the present invention the wire-sides of the substrates,i.e. the sides of the webs located against the wire during themanufacturing of the substrates, are positioned against one another inthe final E12 filter medium. In accordance with yet another variation ofthe thirteenth preferred embodiment of the present invention thewire-side of one web is positioned against the side away from the wireof the other web in the final E12 filter medium. In accordance with afurther variation of the thirteenth preferred embodiment of the presentinvention the wire-sides of the webs are positioned as the outer facesof the filter medium, i.e. the sides facing away from the wire in theirmanufacturing stage are being positioned against one another in thefinal E12 filter medium. When using the filter medium, it may be used asthe wire side positioned as the dirty side, i.e. against the dirty fluidflow to the filter medium or the wire side as the clean side, i.e.facing away from the dirty fluid flow.

As an exemplary E12 product may be mentioned a filter medium comprisinga pre-filter substrate and a fine-filter substrate. The pre-filtersubstrate is manufactured in accordance with the variation of the abovediscussed seventh preferred embodiment of the present invention by usinga three-layer headbox provided with two different furnishes such thatthe surface layers are of similar furnish and the center layer of thedifferent furnish. The pre-filter substrate comprises 33 wt-% PET-fibers having a diameter of 6 micrometer, 10 wt-% PET- fibers having adiameter of 10 micrometer, 13 wt-% micro-glass fibers having an averagediameter between 0.1 and 0.6 micrometer, 5 wt-% micro-glass fibershaving an average diameter between 0.1 and 1.0 micrometer, 34 wt-%bi-component fibers having an average diameter between 8 and 25micrometer, and 5 wt-% binder or meltable fibers. The pre-filtersubstrate has an MD-tensile strength of the order of 600-700 N/m, theCD- tensile strength of the order of 400-500 N/m, and the mean flow poresize varying between 8 and 10 micrometer.

The fine filter substrate comprises 28 wt-% PET-fibers having an averagediameter between 10 and 15 micrometers, 56 wt-% micro-glass fibershaving an average diameter between 0.1 and 1.0 micrometer, 9 wt-%micro-glass fibers having an average diameter between 1 and 3micrometer, and 7 wt-% binder or meltable fibers. The fine-filtersubstrate has the MD-tensile strength of the order of 1800-1900 N/m, theCD-tensile strength of the order of 800-900 N/m, the Mullen burststrength (both wet and dry) within 50-100 kPa, and the mean flow poresize varying between 5 and 6 micrometer.

The MD-tensile strength for the laminated E12 product is of the order of2000-2100 N/m, the CD-tensile strength of the order of 900-1000 N/m, theMullen burst strength (both wet and dry) within 200-400 kPa, and themean flow pore size between 3 and 4 micrometer.

EXAMPLE 3—H13 FILTER MEDIUM

The filter medium in accordance with a fifteenth preferred embodiment ofthe present invention is designed to fulfil the requirements set by BSEN 1822 for an H13 filter medium, i.e. the DEHS efficiency is at least99.95%. The H13 filter medium of the present invention has a total basisweight of 50 to 200 g/m², preferably from 80 to 150 g/m², morepreferably 100-130 g/m². The H13 filter medium of the present inventioncontains a pre-filter substrate with a basis weight of 20 g/m²-80 g/m²,preferably 30 g/m²-70 g/m², more preferably 35-50 g/m², corresponding tothe pre-filter substrate discussed already above, and a fine-filtersubstrate having a basis weight of 30 g/m²-100 g/m², preferably from50-90 g/m², more preferably about 60 g/m².

The fine-filter substrate is formed of synthetic fibers like forinstance PET, PP, PBT, PA, acrylic, lyocell, rayon, aramid, nylon,polyolefin, polyester, or viscose fibers; micro-glass fibers, and asecond binder. The second binder comprises at least one of bicomponentfibers, multicomponent fibers, fully meltable fibers and binder resins.The binder resins comprise at least one of acrylic resins, alkyd resins,butadiene resins, epoxy resins, melamine formaldehyde resins, melamineresins, methacrylic resins, phenolic resins, polyamide resins,polyaromatic resins, polyester resins, polyethylene vinyl acetatecopolymer resins, polyurea resins, polyurethane resins, polyvinylacetate resins, polyvinyl acetyl resins, polyvinyl alcohol resins,polyvinylchloride (PVC) resins, resorcinol resins, styrene resins, ureaphenol resins, urea formaldehyde resins, vinyl acetate resins and vinylchloride resins or any combination thereof. The fine-filter substratecontains 10-80 wt-%, preferably from 50 to 80 wt-%, more preferablyabout 60-70 wt-% micro-glass fibers; 20-60 wt-%, preferably 15-40 wt-%,more preferably about 20-35 wt-% synthetic fibers and 2-15 wt-%,preferably about 3-10 wt-% second binder (including powder, granule orliquid binders). As to the second binder it should be understood thatalso bi-or multicomponent fibers may be used. However, in such a casethe amount of both the synthetic fibers and the binder should bereduced. For example by replacing one half of the synthetic fibers withbi- or multicomponent fibers also the amount of the binder could beabout halved. The micro-glass fibers have an average diameter of 0.01 to5 micrometer, a majority, i.e. more than 60% of the fibers having anaverage diameter between 0.1 to 2 micrometers. However, as themicro-glass fibers are commercially available in various grades (0.4,0.6, 0.8 etc. or 0.1-0.6 micrometers, 0.1-0.8 micrometers, 0.1-1micrometers, 0.5-5 micrometers, etc., referring to the average diameterof the fibers in the grade) the micro-glass used in the furnish may beof only one grade or of a mixture of at least two grades.

In accordance with a prefered variation of the fifteenth embodiment ofthe present invention, micro-glass from at least two different grade, ordiameter ranges, is used, i.e. about 3-10 wt-% of the micro-glassoriginating from a grade having an average diameter of 1 to 3micrometers, and the rest (97-90 wt-%) originating from a grade havingan average diameter of 0.1 to 1.0 micrometer. Preferably, the syntheticfibers are PET-fibers having an average diameter of 10-15 micrometers.

The web for the fine-filter substrate may be produced with any knownpapermaking-type machines such as commercially available fourdrinier,wire cylinder, Stevens former, rotoformer, inver former, venti former,and inclined delta former. After the fibrous web for the fine-filtersubstrate is dried to an appropriate dryness second binder, unless addedto the furnish for making the fine-filter substrate, is applied on theweb by spraying, gravure coating, impregnation, scattering etc.whereafter the web is heated to bind the various fibers to one another.

Finally, the fine-filter substrate may be provided, if desired, withmeans for making it repellent against water, humidity, hydro carbonsand/or alcohols. Such chemical means may be formed of or comprisesilicones or fluorochemicals. The chemicals may be arranged on one ormore of the fibers forming the fine-filter substrate before the formingof the furnish, or the furnish may be provided with respective liquidchemicals, or the web may be provided with the chemicals by means of,for instance, impregnation or coating.

For producing the H13 filter medium the two substrates, i.e. thepre-filter substrate and the fine-filter substrate are laminated to oneanother by means of applying, for instance by gravure coating, curtaincoating or spraying 3-10 g/m², preferably about 5 g/m², appropriatethird binder, i.e. a hot melt binder or adhesive, like for instanceethylene-vinyl acetate (EVA) copolymers, polyolefins (PO), atacticpolypropylene (PP or APP), polybutene-1, oxidized polyethylene,polyamides and polyesters, styrene block copolymers (SBC), preferably apolyurethane hot melt glue on one of the substrates, and thereaftercombining the substrates to form the pulse-cleanable filter medium.Another way to laminate the two substrates together is to heat at leastone of the substrates to make the thermoplastic already therein tacky,and thereafter to combine the substrates to form the pulse-cleanablefilter medium. The thermoplastic here refers to either meltable or bi-or multicomponent fibers.

In accordance with another variation of the fifteenth preferredembodiment of the present invention the wire-sides of the substrates,i.e. the sides of the webs located against the wire during themanufacturing of the substrate, are positioned against one another inthe final H13 filter medium. In accordance with yet another variation ofthe fifteenth preferred embodiment of the present invention thewire-side of one substrate is positioned against the side away from thewire of the other substrate in the final H13 filter medium. Inaccordance with a further variation of the fifteenth preferredembodiment of the present invention the wire-sides of the webs arepositioned as the outer faces of the filter medium, i.e. the sidesfacing away from the wire in their manufacturing stage are beingpositioned against one another in the final H13 filter medium. Whenusing the filter medium, it may be used as the wire side positioned asthe dirty side, i.e. against the dirty fluid flow to the filter mediumor the wire side as the clean side, i.e. facing away from the dirtyfluid flow.

As an exemplary H13 product may be mentioned a filter medium comprisinga pre-filter substrate and a fine-filter substrate. The pre-filtersubstrate is manufactured in accordance with the variation of the abovediscussed seventh preferred embodiment of the present invention by usinga three-layer headbox provided with two different furnishes such thatthe surface layers are of similar furnish and the center layer of thedifferent furnish. The pre-filter substrate comprises 33 wt-% PET-fibershaving a diameter of 6 micrometer, 10 wt-% PET- fibers having a diameterof 10 micrometer, 13 wt-% micro-glass fibers having an average diameterbetween 0.1 and 0.6 micrometer, 5 wt-% micro-glass fibers having anaverage diameter between 0.1 and 1.0 micrometer, 34 wt-% bi-componentfibers having an average diameter between 8 and 25 micrometer, and 5wt-% binder or meltable fibers. The pre-filter substrate has the MD-tensile strength of the order of 600-700 N/m, the CD- tensile strengthof the order of 400-500 N/m, and the mean flow pore size varying between8 and 10 micrometer.

The fine filter substrate comprises 28 wt-% PET-fibers having an averagediameter between 10 and 15 micrometers, 62 wt-% micro-glass fibershaving an average diameter between 0.1 and 1.0 micrometer, 3 wt-%micro-glass fibers having an average diameter between 1 and 3micrometer, and 7 wt-% binder or meltable fibers. The fine-filtersubstrate has the MD- tensile strength of the order of 1800-1900 N/m,the CD-tensile strength of the order of 800-900 N/m, the Mullen burststrength (both wet and dry) within 50-100 kPa, and a mean flow pore sizevarying between 4 and 5 micrometer.

The MD-tensile strength for the laminated H13 product is of the order of2000-2100 N/m, the CD-tensile strength of the order of 900-1000 N/m, theMullen burst strength (both wet and dry) within 200-400 kPa, and themean flow pore size between 2.5 and 3.5 micrometer.

EXAMPLE 4—H14 FILTER MEDIUM

The filter medium in accordance with a sixteenth preferred embodiment ofthe present invention is designed to fulfil the requirements set by BSEN 1822 for an H14 filter medium, i.e. the DENS efficiency is at least99.995%. The H14 filter medium of the present invention has a totalbasis weight of 50 to 200 g/m², preferably from 80 to 150 g/m², morepreferably 100-130 g/m². The H14 filter medium of the present inventioncontains a pre-filter substrate with a basis weight of 20 g/m²-80 /m²,preferably 30 g/m² -70 g/m², more preferably 35-50 g/m², correspondingto the pre-filter substrate already above, and a fine-filter substratehaving a basis weight of 30 g/m²-180 g/m², preferably from 50-120 g/m²,more preferably 60-90 g/m².

The fine-filter substrate is formed of synthetic fibers like forinstance PET, PP, PBT, PA, acrylic, lyocell, rayon, aramid, nylon,polyolefin, polyester, or viscose fibers; micro-glass fibers, and abinder, i.e. at least one of, for instance, acrylic resins, alkydresins, butadiene resins, epoxy resins, melamine formaldehyde resins,melamine resins, methacrylic resins, phenolic resins, polyamide resins,polyaromatic resins, polyester resins, polyethylene vinyl acetatecopolymer resins, polyurea resins, polyurethane resins, polyvinylacetate resins, polyvinyl acetyl resins, polyvinyl alcohol resins,polyvinylchloride (PVC) resins, resorcinol resins, styrene resins, ureaphenol resins, urea formaldehyde resins, vinyl acetate resins and vinylchloride resins or any combination thereof. The fine-filter substratecontains 10-80 wt-%, preferably from 50 to 80 wt-%, preferably about60-70 wt-% micro-glass fibers, 20-60 wt-%, preferably 15-40 wt-%, morepreferably about 20-35 wt-% synthetic fibers and 2-15 wt-%, preferablyabout 3-10 wt-% second binder (including powder, granule or liquidbinders). As to the second binder it should be understood that alsobi-or multicomponent fibers may be used. However, in such a case theamount of both the synthetic fibers and the binder should be reduced.For example by replacing one half of the synthetic fibers with bi- ormulticomponent fibers also the amount of the binder could be abouthalved. The micro-glass fibers have an average diameter of 0.01 to 5micrometer, a majority, i.e. more than 60% of the fibers having anaverage diameter between 0.1 to 2 micrometers. However, as themicro-glass fibers are commercially available in various grades (0.4,0.6, 0.8 etc. or 0.1-0.6 micrometers, 0.1-0.8 micrometers, 0.11micrometers, 0.5-5 micrometers, etc., referring to the average diameterof the fibers in the grade) the micro-glass used in the furnish may beof only one grade or of a mixture of at least two grades.

In accordance with a preferred variation of the sixteenth embodiment ofthe present invention, micro-glass from at least two different sources,or diameter ranges, is used, i.e. about 25-40 wt-% of the micro-glassoriginating from a source having an average diameter of 0.1 to 0.8micrometers, and the rest (75-60 wt-%) originating from a source havingan average diameter of 0.1 to 1.0 micrometer. Preferably, the syntheticfibers are PET-fibers having an average diameter of 10 to 15micrometers.

The web for the fine-filter substrate may be produced with any knownpapermaking-type machines such as commercially available fourdrinier,wire cylinder, Stevens former, rotoformer, inver former, venti former,and inclined delta former. After the fibrous web for the fine-filtersubstrate is dried to an appropriate dryness second binder is, unlessadded to the furnish for making the fine-filter substrate, applied onthe web by spraying, gravure coating, impregnation, scattering etc.whereafter the web is heated to bind the various fibers to one another.

Finally, the fine-filter substrate may be provided, if desired, withmeans for making it repellent against water, humidity, hydro carbonsand/or alcohols. Such chemical means may be formed of or comprisesilicones or fluorochemicals. The chemicals may be arranged on one ormore of the fibers forming the fine-filter substrate before the formingof the furnish, or the furnish may be provided with respective liquidchemicals, or the web may be provided with the chemicals by means of,for instance, impregnation or coating.

For producing the H14 filter medium the two substrates, i.e. thepre-filter substrate and the fine-filter substrate are laminated to oneanother by means of applying, for instance by gravure coating, curtaincoating or spraying 3-10 g/m², preferably about 5 g/m², appropriatethird binder, i.e. a hot melt binder or adhesive, like for instanceethylene-vinyl acetate (EVA) copolymers, polyolefins (PO), atacticpolypropylene (PP or APP), polybutene-1, oxidized polyethylene,polyamides and polyesters, styrene block copolymers (SBC), preferably apolyurethane hot melt glue on one of the substrates, and thereaftercombining the substrates to form the pulse-cleanable filter medium.

In accordance with a another variation of the sixteenth preferredembodiment of the present invention the wire-sides of the webs, i.e. thesides of the webs located against the wire during the manufacturing ofthe webs, are positioned against one another in the final H14 filtermedium. In accordance with yet another variation of the sixteenthpreferred embodiment of the present invention the wire-side of one webis positioned against the side away from the wire of the other web inthe final H14 filter medium. In accordance with a further variation ofthe sixteenth preferred embodiment of the present invention thewire-sides of the webs are positioned as the outer faces of the filtermedium, i.e. the sides facing away from the wire in their manufacturingstage are being positioned against one another in the final H14 filtermedium. When using the filter medium, it may be used as the wire sidepositioned as the dirty side, i.e. against the dirty fluid flow to thefilter medium or the wire side as the clean side, i.e. facing away fromthe dirty fluid flow. As an exemplary H14 product may be mentioned afilter medium comprising a pre-filter substrate and a fine-filtersubstrate. The pre-filter substrate is manufactured in accordance withthe variation of the above discussed seventh preferred embodiment of thepresent invention by using a three-layer headbox provided with twodifferent furnishes such that the surface layers are of similar furnishand the center layer of the different furnish. The pre-filter substratecomprises 33 wt-% PET- fibers having a diameter of 6 micrometers, 10wt-% PET- fibers having a diameter of 10 micrometers, 13 wt-%micro-glass fibers having an average diameter between 0.1 and 0.6micrometers, 5 wt-% micro-glass fibers having an average diameterbetween 0.1 and 1.0 micrometers, 34 wt-% bicomponent fibers having anaverage diameter between 8 and 25 micrometers, and 5 wt-% binder ormeltable fibers. The pre-filter substrate has the MD- tensile strengthof the order of 600-700 N/m, the CD-tensile strength of the order of400-500 N/m, and the mean flow pore size varying between 8 and 10micrometers.

The fine-filter substrate comprises 28 wt-% PET- fibers having anaverage diameter between 10 and 15 micrometers, 23 wt-% micro-glassfibers having an average diameter between 0.1 and 0.8 micrometer, 42wt-% micro-glass fibers having an average diameter between 0.1 and 1.0micrometer, and 7 wt-% binder or meltable fibers. The fine-filtersubstrate has the MD- tensile strength of the order of 1800-1900 N/m,the CD- tensile strength of the order of 800-900 N/m, the Mullen burststrength (both wet and dry) within 50-100 kPa, and the mean flow poresize varying between 3 and 4 micrometers.

The laminated H14 product has the MD- tensile strength of the order of2000-2100 N/m, the CD-tensile strength of the order of 900-1000 N/m, theMullen burst strength (both wet and dry) within 200-400 kPa, and themean flow pore size between 1.5 and 2.5 micrometers.

EXAMPLE 5

The filter medium in accordance with a seventeenth preferred embodimentof the present invention has a total basis weight of 80 to 140 g/m²,preferably from 100 to 130 g/m², more preferably about 110-120 g/m². Thefilter medium of the present invention contains a pre-filter substratewith a basis weight of 30 g/m²-60 g/m², preferably 35 g/m²-50 g/m², morepreferably 40-50 g/m², corresponding to those discussed already aboveunder heading ‘Pre-filter substrate’, and a fine-filter substrate havinga basis weight of 30 g/m²-100 g/m², preferably from 50-90 g/m², morepreferably about 60-70 g/m².

Unlike the earlier discussed embodiments the fine-filter substrate isapplied directly on the pre-filter substrate, i.e. without manufacturinga web of its own as the fine-filter substrate. Thus, after thepre-filter substrate is prepared, on one surface thereof, i.e. eitherthe wire-side surface thereof or the opposite surface, is applied, forinstance by gravure coating, curtain coating or spraying, 3-10 g/m²,preferably about 5 g/m², appropriate third binder, like for instanceethylene-vinyl acetate (EVA) copolymers, polyolefins (PO), atacticpolypropylene (PP or APP), polybutene-1, oxidized polyethylene,polyamides and polyesters, styrene block copolymers (SBC), preferably apolyurethane hot melt glue on the pre-filter substrates, and thereafterthe fine-filter material, i.e. at least 50-100 wt-% of the fine-filtersubstrate material, preferably 70-100 wt-%, more preferably 85-100 wt%micro-glass is applied on the binder layer. Together with micro-glass0-50 wt-%, preferably 0-10 wt-%, more preferably 2-6 wt-% additionalsecond binder may be applied. After a desired amount of fine-filtermaterial is applied on the pre-filter substrate the two substrates arecombined to form the pulse-cleanable filter medium.

As to making the fine-filter substrate repellent against water,humidity, hydro carbons and/or alcohols by means of, for instance,silicones or fluorochemicals the fibers forming the fine-filtersubstrate may be treated with the chemicals before the forming of thefiber mix for the fine-filter substrate, or the fiber mix may beprovided with respective liquid chemicals, or the entire pack of the twosubstrates may be provided with the chemicals by means of, for instance,impregnation or coating.

The filter medium of the present invention may be used in engine orhydraulic oil filters, in intake air filters for gas turbines orinternal combustion engines and in HVAC- or APC-filters, just to name afew options without any intention of limiting the use of the inventionto the listed applications. Filtration systems in which the filtrationmedium as described herein may be employed are well known in the art.For example, the filter medium can be pleated to form a filter elementwhich may be removable or disposable, i.e. in the filtration system, thefilter element may be regularly changed when necessary. In all suchfilters the filter medium is placed in either pleated, corrugated,cylindrical or planar configuration and in cartridge, panel or bag formin a casing arranged on the flow path of the fluid to be filtered. In apreferred embodiment, the filter medium of the present invention is usedin a way such that the fluid to be filtered exits the filter medium atthe side of the medium upon which the fine filter substrate isallocated.

While the invention has been described herein by way of examples inconnection with what are, at present, considered to be the mostpreferred embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments, but is intended to cover variouscombinations or modifications of its features, and several otherapplications included within the scope of the invention, as defined inthe appended claims. The detailed features mentioned in connection withany embodiment above may be used in connection with another embodimentwhenever such combination is technically feasible.

What is claimed is:
 1. A filter medium for filtering a fluid comprising, a prefilter substrate comprising a fibrous nonwoven web having at least a first layer, a second layer, and a third layer; wherein the first layer, the second layer, and the third layer are entangled to one another; the second layer is located between the first layer and the third layer; the first layer and the third layer each independently comprise at least a first binder and first synthetic fibers; and the second layer comprises the first binder, micro-glass fibers, and second synthetic fibers; and a fine-filter substrate comprises a second binder and at least one of third synthetic fibers and first inorganic fibers; and wherein the prefilter substrate is laminated by means of a hot melt binder or an adhesive to the fine-filter substrate to form a laminated filter medium, a first mean flow pore size of the fine-filter substrate ranges from 3 to 10 micrometers, and second mean flow pore size of the laminated filter medium ranges from 1.5 to 6 micrometers.
 2. The filter medium of claim 1, wherein the first binder in the first, second, and third layer independently comprises at least one of multicomponent fibers, fully meltable fibers, and first binder resin.
 3. The filter medium of claim 1, wherein at least one of the first layer, the second layer, and the third layer further comprises second inorganic fibers.
 4. The filter medium of claim 1, wherein the first layer and the third layer each independently comprise between 5 and 30 wt.% first synthetic fibers and between 0 and 10 wt.% first binder with all components of the first layer totaling 100 wt.% of the first layer, and all components of the third layer totaling 100 wt.% of the third layer.
 5. The filter medium of claim 1, wherein the second layer comprises between 15 and 25 wt.% second synthetic fibers, between 10 and 30 wt.% micro-glass fibers, and between 0 and 10 wt.% first binder with all components of the second layer totaling 100 wt.% of the second layer.
 6. The filter medium of claim 1, wherein the fine-filter substrate comprises between 20 and 60 wt.% third synthetic fibers, between 10 and 80 wt.% first inorganic fibers, and between 2 and 15 wt.% second binder with all components of the fine-filter substrate totaling 100 wt.% of the fine-filter substrate.
 7. The filter medium of claim 1, wherein the fine-filter substate comprises chop-strand glass fibers having an average diameter of between 1 and 30 micrometers.
 8. The filter medium of claim 1, wherein the third synthetic fibers comprise at least one of PET, PP, PBT, PA, lyocell, rayon, aramid, nylon, polyolefin, polyester, viscose, and acrylic fibers, and the third synthetic fibers have an average diameter between 0.1 and 100 micrometers.
 9. The filter medium of claim 1, wherein the first inorganic fibers have an average diameter between 0.01 and 5 micrometers.
 10. The filter medium of claim 1, wherein the prefilter substrate and the fine-filter substrate are bonded together by a third binder selected from the group consisting of ethylene-vinyl acetate (EVA) copolymers, polyolefins (PO), atactic polypropylene (PP or APP), polybutene-1, oxidized polyethylene, polyamides and polyesters, styrene block copolymers (SBC), polyurethane hot melt glue, and combinations thereof.
 11. The filter medium of claim 1, having a Mullen burst strength (both wet and dry) of between 200 and 400 kPa.
 12. The filter medium of claim 2, wherein the multicomponent fibers comprising at least one of PET/co-PET, PET/co-PE, PET/co-PP, and PET/co-PA fibers, the multicomponent fibers having a finess of 1 to 15 den.
 13. The filter medium of claim 2, wherein the second binder comprises a second binder resin, and the first binder resin and the second binder resin are selected from the group consisting of acrylic resins, alkyd resins, butadiene resins, epoxy resins, melamine formaldehyde resins, melamine resins, methacrylic resins, phenolic resins, polyamide resins, polyaromatic resins, polyester resins, polyethylene vinyl acetate copolymer resins, polyurea resins, polyurethane resins, polyvinyl acetate resins, polyvinyl acetyl resins, polyvinyl alcohol resins, polyvinylchloride (PVC) resins, resorcinol resins, styrene resins, urea phenol resins, urea formaldehyde resins, vinyl acetate resins, vinyl chloride resins, and combinations thereof.
 14. The filter medium of claim 2, wherein the fully meltable fibers are selected from the group consisting of PE (polyethylene) fibers, PP fibers, polyvinyl chloride (PVC) fibers, polyvinyl alcohol (PVA) fibers, and combinations thereof.
 15. The filter medium of claim 3, wherein at least one of: the first synthetic fibers and the second synthetic fibers compared to the third synthetic fibers, and/or the first inorganic fibers compared to the second inorganic fibers are from at least two different diameter grades.
 16. A filter comprising a casing and at least one filter cartridge, filter panel, or filter bag arranged therein; and the at least one filter cartridge, filter panel, or filter bag comprises the filter medium recited in claim
 1. 17. A filter for filtering engine or hydraulic oils comprising the filter medium in accordance with claim 1 wherein the filter medium is used in pleated, corrugated, cylindrical, or planar configuration. 